I believe they said the Spirit dropped at 50 mph after the retrorockets initiated whereas if it didnt have those, it would drop at or around 250-300 mph...thats a mighty tough landing for a balloon to take.
From the website, the estimated speed of the beagle at "bounce-down" was less than 50 mph, even without retros..
I'll bet the parachute tore or something, poor thing went GADUNK.